1. Field of the Invention
This invention relates generally to the field of dispenser cathodes, and specifically to processes used to make improved barium calcium aluminate impregnants for dispenser cathodes.
2. Prior Art
Currently, the processs for fabricating barium calcium aluminate impregnate for dispenser cathodes consists of first mixing together, in a powdered form, the starting materials barium carbonate, calcium carbonate, and aluminum oxide, and then calcining the mixture to drive off the carbon dioxide thereby forming the barium calcium aluminate or impregnant. In this prior art process, nonhomogeneous impregnants frequently result because complete mixing of the powdered starting materials is difficult and also because complete reaction of the starting materials does not always occur due to the relatively large grain size of the starting materials.
Furthermore, the impurity, sulfur, is introduced into the impregnant from the starting materials, thereby resulting in cathode poisoning with subsequent performance degradation.
The ground or powdered impregnant so formed is then placed in contact with the porous tungsten cathode where it is melted into the pores of the cathode at a temperature of approximately 1600.degree. C. Most impregnants consist of two or three crystalling phases that have a relatively wide range between the solidus temperature (initial formation of a liquid phase) and the liquidus temperature (disappearance of the solid phase). This range of melting temperatures results in an impregnant that is nonhomogeneous and is deficient in the higher melting temperature phases. Thus, a substantial disadvantage of the prior art is that the composition of the impregnant can not be controlled during melting thereby resulting in undesirable variations in cathode performance.